1. Field of the Invention
This invention relates to a methane fermentation making use of novel methanogenic bacteria which exhibit high methanogenic efficiency and are highly resistant to oxygen.
2. Description of the Prior Art
A number of processes have been known for the production of methane gas by fermentation. Typical of high speed methane fermentation processes is a process which is described, for example, in the Collection of Prearranged Manuscripts of the Agricultural Chemical Society of Japan, p. 82, 1981. In this process the continuous fermentation operation is effected using a 0.5 m.sup.3 liquefaction tank and a 1.5 m.sup.3 gasification tank under conditions of a total retention time of 8 days and an organics labor of 15 kg/m.sup.3.multidot. day thereby obtaining 300 l of a fermentation gas per kg of the organics having a methane concentration of 70% by volume. In this instance, the temperature conditions of thermophilic range (60.degree. C.) where the high fermentation efficiency is achieved are adopted. The quantity of methane gas generated is 6.3 m.sup.3 /day and the retention times required for the gasification and liquefaction are, respectively, 6 and 2 days. The methane gas formation process in the gasification tank is the rate-limiting step of the whole fermentation process. In other words, in order to enhance the fermentation efficiency, the methane formation process in which methanogenic bacteria take part should proceed at high efficiency. Accordingly, the methanogenic activity of the bacteria is one of the most important factors controlling the efficiency of the whole fermentation process.
The above instance is a continuous fermentation process in which the concentration of the methanogenic bacteria in the gasification tank is held constant during the course of the stationary operation. In the continuous process, a given amount of fresh substrate is fed while withdrawing the same amount of the reactor effluent from the tank, so that methanogenic bacteria increase in an amount corresponding to that of bacteria entrained with the discharged effluent. Accordingly, when the doubling time of methanogenic bacteria is calculated from the charge and discharge rates of fermentation liquor, it is about 100 hours. That is, the increase or multiplication of methanogenic bacteria in the fermentation process is slight in degree, revealing that the methanogenic activity in the stationary phase of growth of methanogenic bacteria is very important for the methane fermentation.
As is clearly described in the Bergey's Manual of Determinative Bacteriology, methanogenic bacteria are very strict anaerobes and will die out within a short time when oxygen is incorporated into the system even in very small amounts. This characteristic of high sensitivity and low resistance to oxygen involves a considerable difficulty in case where the seed of methanogenic bacteria is preserved, subjected to the mass culture and/or transported. Accordingly, methanogenic bacteria to be used in practical applications should desirably be highly resistant to oxygen.